In the exposure process for the fields of manufacturing liquid crystal displays, touch screens, etc., a proximity exposure machine is usually used. When using a proximity exposure machine, the device's detection of the exposure gap between the mask plate and the substrate is critical. The commonly used exposure gap detector is a spectroscopic interferometric detector. The detector is located at a position perpendicular to the substrate, and a spectroscopic spectrum analysis is performed by receiving the reflected light, thereby measuring the exposure gap.